1. Field of the Invention
The present invention relates to novel polyamideimide resins and to a process for preparing them. More specifically, it relates to a novel type of polyamideimide resins having excellent solubility and melt flowability as well as heat resistance by mixing aromatic diamines with aliphatic diamines, and a process for preparing them.
2. Description of the Prior Art
Polyamideimide resins are, in general, amorphous thermoplastic resins which can be prepared by the polycondensation of aromatic tricarboxylic acids (or their derivatives) with aromatic diamines or aromatic diisocyanates. The molecular structures of the polyamideimide resins vary depending on the type of monomers employed, but the typical example of them are represented by the following formula (i) ##STR4## wherein R= ##STR5## prepared by the polycondensation of trimellitic acid (or its derivatives) as an aromatic tricarboxylic acid component with meta-phenylene diamine and diaminodiphenylether as an aromatic diamine component.
Polyamidimide resins of formula (i) are disclosed in U.S. Pat. No. 4,045,407 and Japanese Patent Laid-Open No. 02-18,422. They are transparent and noncrystalline resins which have the following properties:
(1) They have a heat distortion temperature of 278.degree. C. and a long term heat resistance temperature exceeding 200.degree. C. They have a broad usable temperature range from -200.degree. C. to 260.degree. C.;
(2) They have not only high mechanical strength and excellent impact resistance but also good stiffness providing physical property at room temperature of general engineering plastics even at temperature exceeding 200.degree. C.;
(3) They also have creep resistance;
(4) They have a small linear expansion coefficient of 4.times.10.sup.-5 cm/cm. .degree. C., which can be reduced to less than a half by using filler.
(5) They have excellent insulation breakdown strength and volume resistivity, and show inflammability of UL 94 V-O without adding additives.
(6) They have a good wear resistance property by means of the compositing with PTFE and graphite. They are suitable as sliding member under severe circumstance since they have good self-lubricating properties, wear resistance and strength and elasticity even at high temperatures.
(7) They have good chemical resistance and are stable in a hydrocarbon type solvent, though care must be taken in concentrate aqueous alkali solution.
(8) They have good ultraviolet resistance and radiation resistance. The existing resins of formula (i) have excellent heat resistance, but they require melt processing at a temperature as high as 400.degree. C. due to their high melt viscosity. Such processing at high temperature leads to significant thermal decomposition of the resins, and requires specific process equipment resulting in higher processing costs.
To eliminate the disadvantages of the conventional polyamideimide resins, U.S. Pat. No. 4,017,459 suggested the use of diamines wherein a flexible group such as --O-- and --C(CH.sub.3).sub.2 -- is present between two benzene rings, and Japanese Patent Laid-Open Nos. 02-84,404 and 02-97,527 suggested the preparation of injection moldable polyamideimide wherein diamines having a bond such as --O--, --S--, --SO.sub.2 --, --CO-- or --C(CH.sub.3).sub.2 -- between two benzene rings or substituted monomers are employed. Moreover, Japanese Patent Publication Nos. 63-258,927 and 63-256,627 suggested the use of the diamines wherein aromatic diamines are partly replaced by aliphatic diamines. The polyamideimide resins modified as described above have a decreased melt molding temperature owing to the increased chain flexibility, but their heat resistance and mechanical properties are significantly decreased. Therefore, the resins are impractical.
In order to solve the above-mentioned problems, assiduous studies have been made by the inventors of the present application to find improved polyamideimide resins having better melt processability combined with the existing properties including good solubility and general physical properties of the conventional polyamideimide resin. This is to ensure that they can be used as major heat resistance structural materials in advanced industries, for example, the auto, electrical and electronic, and aerospace industries. As a result, the present inventors have now found that by modifying the aromatic diamines of the conventional polyamideimide resins, particularly by choosing isophorone diamine as an aliphatic diamine and then mixing it with aromatic diamines, polyamideimide resins can be produced which have good mechanical properties and processability. The present invention is based on these findings.